This invention relates generally to an ultrasonic device to monitor, measure and control tie bar stresses and strain in machines that use tie bars as a basic part of the design of the machine, such as die casting machines, injection molding machines, and all other machines that use tie bars.
In this device, an ultrasonic system is used to measure stress and strain in the bars, due to tension, bending, or stress imparted to the bars or stresses caused by temperature changes within the die, the platens, or the bars themselves.
This ultrasonic detection system produces an electrical signal, the output of which can be used to automatically adjust an individual tie bar for strains, or in the case of an overload, to sound an alarm, and/or shut down the machine.
Large die casting machines such as those used in the manufacture of aluminum automotive drive train components use a plurality of very large threaded tie bars on which one half of a precision die slides to allow the die halves to open and close during each casting cycle. It will be appreciated that the precision die must be evenly loaded during a casting cycle to insure proper formation and dimensional accuracy of the part being cast and to prevent molten metal from leaking into the seam between the die halves. Once solidified, this metal becomes "flash". Accordingly, in such die casting machines which frequently operate on an almost continuous basis during operation, it is important that the tie bar tension from bar to bar remain within preselected limits to maintain part quality and to prevent tie bar failure from uneven loads, and to prevent excessive wear on dies and the machine itself.
During operation of these machines, even if the initial tie bar tension is accurately provided for the desired operation of the machine, the tension in the several tie bars will subsequently vary. This variation is due largely to the thermal effect of the material introduced into the die. This heating effect can cause the preselected tensile forces on the several tie bars to change dramatically resulting in uneven closing of the die; and it may result in undesirable forces on closing of the die halves. Of course, all of these events can lead to defective product, broken tie bars, and/or cracked dies. In addition, the flash can become coined into the die face, which shortens the die life.
Even a small amount of flash build-up can cause uneven tie bar and die face loading. As the machine cycles, the uneven loading will fatigue the tie bars and warp the die. This can be extremely expensive since a replacement die can incur costs of about $750,000 and a replacement tie bar can cost about $20,000. In addition, other repair costs associated with damage caused by uneven tie bar and die face loading include lost revenue from down time and labor costs incurred during repairs.
Typically, with most tie bar machines, it has been necessary to frequently manually and individually adjust each of the tie bars during operation to assure that the product quality remains relatively constant with changing temperatures, and that tension on the tie bars of the machine remain even. Of course, such adjusting will shut down operation of the machinery and is costly in terms of lost production.
Monitoring systems for conventional tie bar machines are in use, but unfortunately all suffer from certain drawbacks and disadvantages. For example, one method involves connecting strain gauges with associated analog dial readouts to each of the tie bars. However, this system is inadequate because the response time of the analog readouts and the inability to track bending in the tie bars can result in premature failure of the tie bars and/or die. An example of a tie bar monitoring system employing strain gauges associated with an electrical circuit for detecting and controlling tie bar tension is described in U.S. Pat. No. 4,256,166, the entire disclosure of which is incorporated herein by reference.
Still another system for monitoring tie bar tension is disclosed in a paper entitled "The Locking End - Tie Bar Adjustments" by Barry Upton, published in Die Casting Management, Nov.-Dec. 1986, pages 18-22. This system utilizes a plurality of linear variable displacement transformer (LVDT) devices. However, the LVDT system is also inadequate because of the delicate mechanical mechanisms required to implement this system and the difficulty in retrofitting existing machines with this type of tie bar monitoring equipment.